This invention relates to the emulation of the polarisation mode dispersion (PMD) that is found to occur in single mode optical waveguide transmission systems. PMD is a dispersion which is limiting the performance of high bit rate transmission systems. All currently installed single mode transmission optical fibre is found to exhibit at least some measure of birefringence resulting from departures from perfect circular symmetry occasioned in the course of manufacture or by the subsequent application of non-circularly-symmetric stress fields. Such a departure from perfect circular symmetry introduces birefringence, thereby removing mode degeneracy and introducing PMD.
The presence of PMD in a long length of installed transmission fibre produces differential group delay (DGD) in the propagation of light along that fibre. The magnitude of this DGD varies significantly over both time and wavelength. When designing a high speed optical transmission system the impact of PMD, in terms of the DGD that it engenders, needs to be quantified by considering the worst case situation, largest magnitude DGD, likely to occur within the lifetime of the system. This worst case situation is a very low probability event, and so the system designer will generally have to rely upon estimating its DGD, rather than actually measuring it. Estimation requires a model of the behaviour of a typical length of installed fibre. In this context, it may be observed that the behaviour of a reel of fibre in the laboratory does not, and can not be expected to, resemble that of fibre installed in the field.
It is widely accepted that the wavelength dependence properties of the PMD of a long length of installed transmission fibre can be modelled as a concatenation of randomly oriented birefringent elements of different birefringence magnitude. In order to get a realistic representation of the wavelength dependence of the fibre""s behaviour, a very large number of birefringence elements is usually modelled. For a physical emulation, the birefringence elements may at least in principle be constituted by different lengths of birefringent fibre in which birefringence has been deliberately built in, hereinafter referred to as high birefringence fibre. An example of such an emulator is for instance described by C H Prola, Jr. et al. in, xe2x80x98PMD Emulators and Signal Distortion in 2.48 Gb/s UM-DD Lightwave Systemsxe2x80x99. In this particular emulator, the individual lengths of high birefringence fibre are concatenated by splicing them together with random (but fixed) relative orientations of birefringence axes at each splice. In this paper it is particularly stated that xe2x80x98a great number of splicesxe2x80x99 is required xe2x80x98to insure a random-mode coupling and a continuous distribution of polarization time-of-flightsxe2x80x99. This has adverse cost and convenience implications. It would be desirable, other things being equal, to minimise the number of such lengths required to achieve a given level of emulation accuracy. For this, the appropriate choice of relative lengths of high birefringence fibre is likely to be critical. It may then be desirable to provide some mechanism by which the emulator can be caused to accelerate the time exploration of its PMD so as to mimic in minutes, or perhaps a few hours, the PMD behaviour which the real installed fibre may take years to explore.
An object of the present invention is to provide an emulator which closely mimics the PMD properties of an installed long length of fibre by means of a concatenation of a relatively small number of lengths of high birefringence fibre assembled with random relative orientation of their birefringence axes.
According to a first aspect of the present invention, there is provided a method of emulating the first and second order PMD (polarisation mode dispersion) properties of a length of low birefringence optical fibre installed in the field, in which method light is caused to propagate through a concatenation of n lengths of high birefringence optical waveguide, each having two guided modes, respectively a fast mode and a slow mode, and each length presenting a different value of differential group delay, xcfx84r, for light propagating in those modes, where r is a positive integer between 1 and n, wherein the rth length of the concatenation is optically coupled with the (r+1)th length in a manner providing cross coupling of the modes so that only cos2 xcex8r of the power propagating in each mode (fast or slow) of the rth length is launched into the same mode (fast or slow) of the (r+1)th length while the remaining sin2 xcex8r of the power is launched into the opposite mode (slow or fast), wherein the values of xcex8r for r=1 to r=(nxe2x88x921) have a substantially random distribution within the range 0 to xcfx80/2, and wherein said lengths and their differential group delays satisfy the following five criteria (i) to (v):
5xe2x89xa6nxe2x89xa630xe2x80x83xe2x80x83(i)                     0.2        ≤                              τ            range                                ⟨                          DGD                              R                ⁢                                  xe2x80x83                                ⁢                M                ⁢                                  xe2x80x83                                ⁢                S                                      ⟩                          ≤        0.4                            (        ii        )            xe2x80x83xcfx84min less than xcfx84rangexe2x80x83xe2x80x83(iii)                                           τ            _                                ⟨                          DGD                              R                ⁢                                  xe2x80x83                                ⁢                M                ⁢                                  xe2x80x83                                ⁢                S                                      ⟩                          ≤        0.3                            (        iv        )            
and
"sgr"xcfx84 less than {overscore (xcfx84)}xe2x80x83xe2x80x83(v)
where the expected root mean square DGD (differential group delay) of the concatenation is                               ∑                      r            =            1                                r            =            n                          ⁢                  xe2x80x83                ⁢                  τ          r          2                      =          ⟨              DGD                  R          ⁢                      xe2x80x83                    ⁢          M          ⁢                      xe2x80x83                    ⁢          S                    ⟩        ,
the mean DGD of the lengths is                               ∑                      r            =            1                                r            =            n                          ⁢                  xe2x80x83                ⁢                  τ          r                    n        =          τ      _        ,
the DGD values of the lengths respectively with the maximum and minimum DGDs are xcfx84max and xcfx84min, the range of DGD values is xcfx84range=xcfx84maxxe2x88x92xcfx84min, and the standard deviation of the DGD values is "sgr"xcfx84.
According to a second aspect of the present invention, there is provided a method of emulating the first and second order PMD (polarisation mode dispersion) properties of a length of low birefringence optical fibre installed in the field, in which method light is caused to propagate through a concatenation of n lengths of high birefringence optical waveguide, wherein 5xe2x89xa6nxe2x89xa630, each having two guided modes, respectively a fast mode and a slow mode, and each presenting a different value of differential group delay, xcfx84, for light propagating in those two modes, wherein the rth length of the concatenation, where r is a positive integer between 1 and n, is optically coupled with the (r+1)th length in a manner providing cross coupling of the modes so that only cos2 xcex8r of the power propagating in each mode (fast or slow) of the rth length is launched into the same mode (fast or slow) of the (r+1)th length while the remaining sin2 xcex8r of the power is launched into the opposite mode (slow or fast), wherein the values xcex8r for r=1 to r=(nxe2x88x921) have a substantially random distribution within the range 0 to xcfx80/2, wherein said differential group delay values are substantially aligned with members of the series xcfx84=qs, where q is a constant common to all members of the series, and s is a different integer for each member of the series.
According to a third aspect of the present invention, there is provided a method of emulating the first and second order PMD (polarisation mode dispersion) properties of a length of low birefringence optical fibre installed in the field, in which method light is caused to propagate through a concatenation of n lengths of high birefringence fibre, each presenting a different value of differential group delay, xcfx84r, where r is a positive integer between 1 and n, and each connected with its adjacent lengths in the concatenation with birefringence axes unaligned relative orientation, wherein said lengths satisfy the following five criteria (i) to (v):
5xe2x89xa6nxe2x89xa630xe2x80x83xe2x80x83(i)                     0.2        ≤                              τ            range                                ⟨                          DGD                              R                ⁢                                  xe2x80x83                                ⁢                M                ⁢                                  xe2x80x83                                ⁢                S                                      ⟩                          ≤        0.4                            (        ii        )            xe2x80x83xcfx84min less than xcfx84rangexe2x80x83xe2x80x83(iii)
                                          τ            _                                ⟨                          DGD                              R                ⁢                                  xe2x80x83                                ⁢                M                ⁢                                  xe2x80x83                                ⁢                S                                      ⟩                          ≤        0.3                            (        iv        )            
and
"sgr"xcfx84 less than {overscore (xcfx84)}xe2x80x83xe2x80x83(v)
where the expected root mean square DGD (differential group delay) of the concatenation is                               ∑                      r            =            1                                r            =            n                          ⁢                  xe2x80x83                ⁢                  τ          r          2                      =          ⟨              DGD                  R          ⁢                      xe2x80x83                    ⁢          M          ⁢                      xe2x80x83                    ⁢          S                    ⟩        ,
the mean DGD of the lengths is                               ∑                      r            =            1                                r            =            n                          ⁢                  xe2x80x83                ⁢                  τ          r                    n        =          τ      _        ,
the DGD values of the lengths respectively with the maximum and minimum DGDs are xcfx84max and xcfx84min, the range of DGD values is xcfx84range=xcfx84maxxe2x88x92xcfx84min, and the standard deviation of the DGD values is "sgr"xcfx84.
According to a fourth aspect of the present invention, there is provided a method of emulating the first and second order PMD (polarisation mode dispersion) properties of a length of low birefringence optical fibre installed in the field, in which method light is caused to propagate through a concatenation of n lengths of high birefringence optical waveguide, wherein 5xe2x89xa6nxe2x89xa630, each presenting a different value of differential group delay, xcfx84, and each connected with its adjacent lengths in the concatenation with birefringence axes unaligned substantially random relative orientation, wherein said differential group delay values are substantially aligned with members of the series xcfx84=qs, where q is a constant common to all members of the series, and s is a different integer for each member of the series.
According to a fifth aspect of the present invention, there is provided an emulator for emulating the first and second order PMD (polarisation mode dispersion) properties of a length of low birefringence optical fibre installed in the field, the emulator including a concatenation of n lengths of high birefringence optical waveguide through which light is caused to propagate, each length having two guided modes, respectively a fast mode and a slow mode, and each presenting a different value of differential group delay, xcfx84r, for light propagating in those modes, where r is a positive integer between 1 and n, wherein the rth length of the concatenation is optically coupled with the (r+1)th length in a manner providing cross coupling of the modes so that only cos2 xcex8r of the power propagating in each mode (fast or slow) of the rth length is launched into the same mode (fast or slow) of the (r+1)th length while the remaining sin2 xcex8r of the power is launched into the opposite mode (slow or fast), wherein the values of xcex8r for r=1 to r=(nxe2x88x921) have a substantially random distribution within the range 0 to xcfx80/2, and wherein said lengths and their differential group delays satisfy the following five criteria (i) to (v):
5xe2x89xa6nxe2x89xa630xe2x80x83xe2x80x83(i)                     0.2        ≤                              τ            range                                ⟨                          DGD                              R                ⁢                                  xe2x80x83                                ⁢                M                ⁢                                  xe2x80x83                                ⁢                S                                      ⟩                          ≤        0.4                            (        ii        )            xe2x80x83xcfx84min less than xcfx84rangexe2x80x83xe2x80x83(iii)                                           τ            _                                ⟨                          DGD                              R                ⁢                                  xe2x80x83                                ⁢                M                ⁢                                  xe2x80x83                                ⁢                S                                      ⟩                          ≤        0.3                            (        iv        )            
and
"sgr"xcfx84 less than {overscore (xcfx84)}xe2x80x83xe2x80x83(v)
where the expected root mean square DGD (differential group delay) of the concatenation is                               ∑                      r            =            1                                r            =            n                          ⁢                  xe2x80x83                ⁢                  τ          r          2                      =          ⟨              DGD                  R          ⁢                      xe2x80x83                    ⁢          M          ⁢                      xe2x80x83                    ⁢          S                    ⟩        ,
the mean DGD of the lengths is                               ∑                      r            =            1                                r            =            n                          ⁢                  xe2x80x83                ⁢                  τ          r                    n        =          τ      _        ,
the DGD values of the lengths respectively with the maximum and minimum DGDs are xcfx84max and xcfx84min, the range of DGD values is xcfx84range=xcfx84maxxe2x88x92xcfx84min, and the standard deviation of the DGD values is "sgr"xcfx84.
According to a sixth aspect of the present invention, there is provided an emulator for emulating the first and second order PMD (polarisation mode dispersion) properties of a length of low birefringence optical fibre installed in the field, the emulator including a concatenation of n lengths of high birefringence optical waveguide through which light is caused to propagate, wherein 5xe2x89xa6nxe2x89xa630, each having two guided modes, respectively a fast mode and a slow mode, and each presenting a different value of differential group delay, xcfx84, for light propagating in those two modes, wherein the rth length of the concatenation, where r is a positive integer between 1 and n, is optically coupled with the (r+1)th length in a manner providing cross coupling of the modes so that only cos2 xcex8r of the power propagating in each mode (fast or slow) of the rth length is launched into the same mode (fast or slow) of the (r+1)th length while the remaining sin2 xcex8r of the power is launched into the opposite mode (slow or fast), wherein the values of xcex8r for r=1 to r=(nxe2x88x921) have a substantially random distribution within the range 0 to xcfx80/2, wherein said differential group delay values are substantially aligned with members of the series xcfx84=qs, where q is a constant common to all members of the series, and s is a different integer for each member of the series.
Preferably the emulator includes means that causes its PMD to vary with time at a much faster rate than would be typical of the fibre that it is emulating, so that in this way it will perform a PMD exploration in hours, or perhaps a few days, that the fibre itself would be liable to take years to explore.
Other features and advantages of the invention will be readily apparent from the following description of preferred embodiments of the invention, from the drawings and from the claims.